1. Field of the Invention
This invention relates to nitride-based optoelectronic devices and a method of fabricating the same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Nitride-based optoelectronics have been extensively studied for fabrication of visible and ultra-violet (UV) light emitting devices. These devices typically have one or more layers of ternary alloys (InGaN, AlGaN, and AlInN), or quaternary alloy (AlInGaN). Continued developments in nitride-based optoelectronic devices have resulted in high-power and high-efficiency light emitting diodes (LEDs) and laser diodes (LDs), especially in the visible spectrum. However, high-power and high-efficient LEDs and LDs in the deep UV (DUV) region of the spectrum (emitting light with less than roughly 360 nm wavelengths) have not been achieved due to the difficulties in the growth, and thus poor material quality, and the absence of a bulk aluminum nitride (AlN) substrate.
For a nitride-based UV light emitting devices with peak emission wavelength (λpeak) less than 360 nm, conventional LEDs and LEDs comprise of multiple AlGaN layers and an AlN buffer layer, which are normally grown on either sapphire or 6H—SiC substrates. Because of this heteroepitaxial growth, AlN or AlGaN buffer layers bear a dislocation density on the order of 1010 cm−2, and the dislocations propagate through the subsequent layers, resulting in poor material quality.
For indium containing alloys such as InGaN, it is commonly acknowledged that the indium clustering provides highly efficient radiative recombination sites for the carriers, and thus the performance of the device is rather insensitive to the dislocations. In contrast, AlGaN-based devices are sensitive to the dislocation density due to the absence of the indium clustering, and therefore the performance of AlGaN-based devices is directly affected by the number of the dislocations.
To reduce the dislocation density, various structures and growth techniques have been studied. For example, a superlattice structure is grown between a buffer layer and cladding layer, in which the superlattice filters out the dislocations propagating from the buffer layer and is also known to relieve the strain built in from the lattice mismatch. This structure improved the device performance of UV LEDs. The growth techniques used in a metal organic chemical vapor deposition (MOCVD), such as a NH3 flow modulated AlN growth, have successfully improved the quality of the AlN buffer layer. Bulk AlN crystals have been achieved by hydride vapor phase epitaxy (HVPE) and physical vapor transport (PVT). See References [1-4].
Even with a high quality AlN buffer layer or a bulk AlN substrate, the AlGaN-based device still suffers from the undesirable quantum-confined Stark effect (QCSE) as long as a device is grown along a c-direction in which a strong spontaneous polarization exists. Lattice mismatch between layers will induce piezoelectric polarization, which could enhance the degree of the polarization. The strong built-in electric fields from the polarizations cause spatial separation between electrons and holes, that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission. The built-in electric field becomes stronger with higher Al composition.
To summarize, conventional AlGaN-based UV light emitting devices suffer from high dislocation density due to the absence of a bulk AN substrate, and from QCSE which reduces the radiative recombination efficiency.
To circumvent the problem of dislocations, AlInGaN quaternary alloys have been introduced in UV light emitting devices, wherein the indium clustering is expected to improve the device performance. It has been shown that the photoluminescence (PL) emission intensity of AlInGaN-based LEDs is approximately one to two orders of magnitude higher than that of AlGaN-based LEDs. [5] However, the internal quantum efficiency (IQE) of AlInGaN-based LEDs is still around 15%, which is significantly lower than that of InGaN (50%-70%). The external quantum efficiency is still too low (˜1%) to realize commercially feasible UV emitting devices [5].